130 Acid–Base, Electrolyte and Renal Emergencies

ACID–BASE DISTURBANCES

3 Clinical manifestations of respiratory acidosis are secondary to the hyper-

capnia. Look for the following:

(i) The patient is usually warm, flushed, sweaty and tachycardic with

‘bounding’ peripheral pulses, from cardiovascular stimulation.

(ii) Acute confusion, mental obtundation, somnolence and

occasionally focal neurological signs from increased cerebral

blood flow, cerebral vasodilation and raised intracranial pressure.

4 The body compensates to reduce acidaemia by minimizing the excretion of

bicarbonate by the kidneys. However, this renal compensatory response is

slow.

(i) There is no time for any significant renal compensatory response

in an acute respiratory acidosis.

(ii) The kidneys are able to retain bicarbonate in chronic respiratory

acidosis lasting over a few days, so the plasma bicarbonate level

rises and the pH returns towards normal.

(iii) The expected compensatory rise in plasma bicarbonate in acute and

chronic respiratory acidosis may be calculated (see Table 3.2, p. 127).

MANAGEMENT

1 Give oxygen and commence assisted ventilation by bag-mask ventilation.

Call for senior emergency department (ED) doctor help and prepare for

emergency endotracheal intubation, or non-invasive ventilation such as

continuous positive airway pressure (CPAP), for instance in acute pul monary

oedema.

2 Correct any reversible underlying disorder, e.g. naloxone for opiate poisoning.

Respiratory alkalosis

DIAGNOSIS

1 A primary acid–base disturbance, associated with increased alveolar venti-

lation and an arterial PaCO 2 of <35 mmHg (4.7 k Pa).

2 Causes include:

(i) Asthma, pneumonia, pulmonary embolus, pulmonary oedema

and pulmonary fibrosis (mediated by intrapulmonary receptors).

(ii) Hypoxia (mediated by peripheral chemoreceptors).

(iii) Centrally induced hyperventilation secondary to respiratory

centre stimulation:

(a) head injury, stroke

Warning: the pulse oximeter may record a normal oxygen saturation in a

patient receiving supplemental oxygen, despite the presence of dangerous

hypercapnoea from hypoventilation.

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